The mechanism of vertebrate ovulation is incompletely understood. Proponents of contractile theories have implicated follicular smooth muscle in mammals and have demonstrated that contractile microfilament-bearing cells play a role in ovulation in lower vertebrates. Recently, we have found in the hamster follicle, not only smo th muscle, but also microfilament-bearing cells. Consequently we asked the question, "What is the role of sm oth muscle and/or microfilament-bearing cells in ovulation in mammals?" Three approaches incorporating transmission electron microscopy and scanning electron microscopy will be used to investigate the role of sm oth muscle and/or microfilament-bearing cells in hamster ovulation. First, the size, number, and distribution of these cells in hamster follicles will be determined at various times prior to and following ovulatiion (TEM). Second, ultrastructural characteristics of these cells will be compared with known smooth muscle cells from hamster oviduct. Third, surface changes during ovulation will be studied (SEM) and correlated with underlying ultrasturctural changes (TEM) in the same follicles. Information from this study should provide a firm basis for knowledgeable interpretation of contractile theories of ovulation.